This invention relates generally to motor control circuits, and more particularly to a system and apparatus for controlling a motor or other load using an H-bridge circuit having self-latching, high side switches.
Bi-directional motors are useful for controlling a wide variety of devices requiring motion. For example, robotic movements are generally controlled using a number of reversible motors to control the movement of the various components. Other examples include movement of satellite disks, solar concentrator arrays that track the sun, and the like. The very nature of a stepper motor requires that it be moved in both the forward and reverse directions.
Directing current through a motor will cause the motor to turn in one direction, but as described above, it is often desirable to turn the motor in the opposite direction. To control a bi-directional motor and resultantly achieve bi-directional movement, an xe2x80x9cH-bridge,xe2x80x9d also referred to as a push-pull motor driver, is often used. The term xe2x80x9cH-bridgexe2x80x9d comes from the circuit configuration itself, as its schematic resembles the letter xe2x80x9cH.xe2x80x9d
Conventional H-bridge configurations have utilized power MOSFET devices for each switch of the H-bridge. One reason is that MOSFET devices can be sized to handle the current demands of the motors being driven, and MOSFET devices are voltage-controlled and draw virtually no gate current. These MOSFET devices must be continuously supplied with a voltage to maintain a state of the transistor switch. Other H-bridge configurations have included all-bipolar designs, such as using bipolar junction transistors (BJT). Such bipolar devices, however, require supplying continuous base currents to continuously close the transistor switch. Further, such bipolar devices generally exhibit large voltage drops across the bipolar output transistors, thereby dissipating a proportionally large amount of heat. This may necessitate the use of heat sinks or other means of cooling the involved circuitry.
Prior art H-bridge circuits often require level shifters and drivers to drive the high side of the H-bridge. This results in greater circuit complexity, while driving up costs and required circuit board real estate. Another significant disadvantage associated with the use of such drivers is the need to power the driver. Either auxiliary power supplies must be used, or some manner of sharing and converting the motor power supply voltage must be implemented. This further adds to the complexity, cost, and circuit area demands.
Therefore, it would be desirable to provide a system and apparatus that does not require continuous control signals to drive high side H-bridge switches and avoids the need for external drivers and associated power supplies, while reducing cost, complexity, and real estate limitations. The present invention provides a solution to these and other problems of the prior art, and offers other advantages over prior art H-bridge configurations.
The present invention relates to an H-bridge system and apparatus for controlling a motor or other multi-directional load using an H-bridge circuit having self-latching, high side switches.
In accordance with one embodiment of the invention, an H-bridge is provided for controlling current through a load, where the H-bridge is coupled between a supply voltage and a reference voltage. The H-bridge includes a forward circuit that is coupled between the supply and reference voltages. The forward circuit includes a low side switch, a high side thyristor, and the load. A forward input signal is received at both the switch and at a control gate of the thyristor. Asserting the forward input signal turns on the low side switch and the high side thyristor, which allows current to flow through the high side thyristor, the low side switch, and the load. A reverse circuit is also provided, which is coupled between the supply and reference voltages. The reverse circuit includes a second low side switch, a second high side thyristor, and the load. A reverse input signal is received at both the second low side switch and at the control gate of the second high side thyristor. Asserting the reverse input signal turns on the second low side switch and the second high side thyristor, which allows current to flow through the second high side thyristor, the second low side switch, and the load.
In accordance with more specific embodiments of the H-bridge according to the invention, an isolation protection circuit may be provided. This isolation circuit is coupled to the gates of the thyristors to isolate the low end switches from a voltage imparted to the control gates of the thyristors, when that particular thyristor has been turned on. In one embodiment, the isolation protection circuit includes a pair of diodes, each one coupled to the control gate of the thyristors. Other specific embodiments include a snubber circuit coupled between the supply voltage and the load to minimize voltage transients caused by back-electromotive force (EMF) or inductive kick back, which occurs upon switching the current direction through the load. One embodiment of a snubber circuit includes a pair of diodes, one for each of the forward and reverse circuits, coupled between the power supply and different load leads.
In accordance with another embodiment of the invention, a polyphase H-bridge is provided for controlling currents through n different loads, where n is greater than one. The polyphase H-bridge is coupled between a supply voltage and a reference voltage, and includes n high side thyristors each having a cathode, an anode, and a control gate. Each of the n loads is coupled between the cathodes of two different thyristors. Also provided is n low side switches, each coupled between one of the thyristors and the reference voltage, where each of the n low side switches includes a control input. Further, n H-bridge input terminals are provided, each coupled to the control gates of the thyristors and the control input of the low side switches across the loads in which current is to flow. Asserting a signal at a corresponding one of the n H-bridge input terminals enables the current to flow through the respective loads.
In accordance with another embodiment of the invention, a system for controlling a motor is provided. The system includes a motor direction control circuit to output forward and reverse direction signals. A hold-off circuit delays assertion of either the forward or reverse direction signals until the other direction signal has been disabled. A DC motor capable of operation in both forward and reverse directions is provided, as is an H-bridge coupled to the DC motor. The H-bridge receives the forward and reverse direction signals from the hold-off circuit, and is coupled between a supply voltage and a reference voltage. The H-bridge includes a forward circuit that is coupled between the supply and reference voltages. The forward circuit includes a low side switch, a high side thyristor, and the load. A forward input signal is received at both the switch and at a control gate of the thyristor. Asserting the forward input signal turns on the low side switch and the high side thyristor, which allows current to flow through the high side thyristor, the low side switch, and the load. A reverse circuit is also provided, which is coupled between the supply and reference voltages. The reverse circuit includes a second low side switch, a second high side thyristor, and the load. A reverse input signal is received at both the second low side switch and at the control gate of the second high side thyristor. Asserting the reverse input signal turns on the second low side switch and the second high side thyristor, which allows current to flow through the second high side thyristor, the second low side switch, and the load.
In accordance with another embodiment of the invention, a self-latching H-bridge circuit for controlling current through a load is provided, where the H-bridge is powered by a power source. The H-bridge includes a first silicon-controlled rectifier (SCR) having an anode, a cathode, and a gate input; where the first SCR is coupled to the power source via the anode. A first switch is coupled between a reference voltage and the cathode of the first SCR through the load, where the first switch includes a first control input. A first buffer is coupled to the first SCR gate input, and the first control input, to turn on the first SCR and the first switch upon output of a forward direction signal. A second SCR includes an anode, a cathode, and a gate input; where the second SCR is coupled to the power source via the anode. A second switch is coupled between the reference voltage and the cathode of the second SCR through the load, where the second switch includes a second control input. The H-bridge also includes a second buffer coupled to the second SCR gate input, and to the second control input, to turn on the second SCR and the second switch upon output of a reverse direction signal.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description. As will be realized, the invention is capable of other and different embodiments, and its details are capable of modification without departing from the scope and spirit of the invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.